water pressure volume vs pipe size

I presently have a 1" water meter that is located 20' from a 14" water main. The pipe connecting the meter to the main is 3/4" The water then travels through an old galvanised 1" pipe 600' to a house with 1/2" copper.There is a 10 ft rise in the height from street to house. 1 bath 1 kitchen (no dishwasher)1 laundry. line pressure at the old house is 155psi.
A new home on the same water meter will add 3 bathrooms and a laundry and kitchen. the rise will be 18' street to second floor. The plumber has suggested I replace the galvanised pipe with PVC. Question Does the size of the water meter line mean that I will gain nothing by increasing the pipe size to 1 1/4" or 1 1/2" PVC or will the friction be reduced in the line and make up some what for the restriction at the meter. The plumber used some calculation to determine that we are one shy of the maximum number of outlets for that meter.

Friction from a smaller pipe, and especially from an older galvanized pipe which could be significantly smaller ID now than when new, should be noticeable. There are standards based on fixture units, how big a supply is required. Just the altitude change would account for a few psi. Other than altitude, when no flow, the pressure would be the same regardless of the pipe size. To maintain pressure, you need good flow, and a larger supply line would help.

The calculation of size of water meter based on fixtures is junk science, or no science at all, and is based on conditions that don't exist. Most of those things were developed when a family of 6 had one bathroom and the shower ran 6 GPM and the toilet required 5 gallons or more of water to flush. Now we have families of 2 to 4 with 3 bathrooms, some of which have double-sink vanities, toilets that require 1.6 gallons per flush, and showers that are limited to 2.5 GPM.

A 1" meter is usually rated for a maximum continuous flow of 25 GPM and peak flow of at least 50 GPM.

Most houses have 5/8" or 3/4" meters rated for 10 or 15 GPM continuous or 30 gallons peak flow.

Six hundred feet of 1 1/4" PVC Schedule 40 PVC pipe will have a pressure loss of about 15 psi in 600 ft. You will lose about 7 psi for 20 GPM in 1 1/2" pipe. You will rarely run 20 GPM and never in 1/2" copper tubing.

You might also consider polyethylene pipe.

If the main pressure is 155 psi at the new house it probably doesn't matter much what size pipe you use. You will have a regulator set at about 75 psi and it makes no difference it you lose the 80 psi in the meter and pipe or in the regulator. You can run a test and see how much flow you can get while the pressure at the house is 75 psi.

Nevertheless, I would replace the pipe with 1 1/4" which will give you good pressure at 20 to 25 GPM. If you want 40 GPM then I suggest 1 1/2" pipe. You won't need 40 GPM ulesss you have a big irrigation system or you want to use it for fire protection.